Shear stress (SS) induces endothelium-dependent vasodilation of humans conductance vessels due in part to the release of NO. However, whether flow-mediated shear stress (FMSS) modulates NO activity and vascular tone in human resistance vessels of normal controls (NC), hypertensives (HT) and hypercholesterolemic (Hchol) patients has not been previously investigated.Methods and Results: Subcutaneous resistance arteries (internal diameter: 201?26 microns) were isolated from gluteal fat biopsies in 9 NC (6 men and 3 women; age: 46+/-8 years; total plasma cholesterol 163+/-26 mg/dl), 12 HT, (5 men and 7 women; age: 50+/-7 years; total plasma cholesterol 171+/-20 mg/dl), and in 12 HChol, (10 men and 2 women; age: 51?7 years; total plasma cholesterol 273+/-21 mg/dl). Arterial preparations were cannulated and perfused in 15 mL chambers oxygenated at 37 C. After preconstriction with norepinephrine (NE; 3x 10-7 M), FMSS was induced by stepwise increasing the flow rate of the perfusate from 1 to 50 L/min. Studies were repeated after 30-minute incubation with the NO synthase inhibitor N -nitro-L-arginine (L-NNA;10-4 M). Arterial internal diameter was measured directly from amplified images and data expressed as percent of NE-induced contraction. In NC, FMSS induced progressive flow rate-dependent vasodilation from 77+/-9% at 1 L/min to 47+/-14% at 50 mcL/min (p<0.0001). L-NNA significantly blunted this response (mean vasodilation before and after L-NNA: 73+/-6% vs. 94+/-9%, respectively; p=0.04). After endothelium removal, there was virtual abolition of flow-mediated vasodilation (mean vasodilation before and after ER: 75+/-5% vs. 97+/-6%, respectively; p<0.001). HT had significant impairment of flow-mediated dilation compared to NC (mean vasodilation: 95+/-6% and 73+/-6%, respectively; p=0.01). L-NNA did not significantly affect shear stress-induced dilation (mean vasodilation before and after L-NNA: 95+/-6% vs. 89+/-5%, respectively; p=0.12). Hchol had preserved flow-mediated vasodilation compared to NC (mean vasodilation 76+/-7% and 73+/-6%, respectively; p=0.56). However, L-NNA did not significantly affected shear stress-induced dilation (mean vasodilation before and after L-NNA: 76+/-7% vs. 82+/-6%, respectively; p=0.50). SNP responses were not significantly different among the three groups (mean vasodilation in NC, HT, and Hchol were 67+/-7%, 73+/-4%, and 66+/-6%, respectively; p=0.58).Conclusion: Shear stress secondary to increases in flow induces endothelium-dependent NO-mediated vasodilation of the normal human microvasculature. This response is blunted in microvessels from hypertensive patients due to reduced activity of endothelial NO. In contrast , the hypercholesterolemic microvasculature has preserved shear stress-induced dilation in spite of diminished NO activity, presumably due to an increased role of other endothelial vasoactive factors. These findings extend previous observations of impaired endothelial microvascular responses to pharmacological agonists to a more physiological, and perhaps more relevant, stimulus for the regulation of vascular tone. At the same time, they emphasize the differences in the mechanisms leading to the vascular abnormalities characteristic of hypertensive and hypercholesterolemic patients.